Benjamin D. Prince

Time- and frequency-dependent model of time-resolved coherent anti-Stokes Raman scattering (CARS) with a picosecond-duration probe pulse

The hybrid femtosecond∕picosecond coherent anti-Stokes Raman scattering (fs∕ps CARS) technique presents a promising alternative to either fs time-resolved or ps frequency-resolved CARS in both gas-phase thermometry and condensed-phase excited-state dynamics applications. A theoretical description of time-dependent CARS is used to examine this recently developed probe technique, and quantitative comparisons of the full time-frequency evolution show excellent accuracy in predicting the experimental vibrational CARS spectra obtained for two model systems. The interrelated time- and frequency-domain spectral signatures of gas-phase species produced by hybrid fs∕ps CARS are explored with a focus on gas-phase N2 vibrational CARS, which is commonly used as a thermometric diagnostic of combusting flows. In particular, we discuss the merits of the simple top-hat spectral filter typically used to generate the ps-duration hybrid fs∕ps CARS probe pulse, including strong discrimination against non-resonant background that often contaminates CARS signal. It is further demonstrated, via comparison with vibrational CARS results on a time-evolving solvated organic chromophore, that this top-hat probe-pulse configuration can provide improved spectral resolution, although the degree of improvement depends on the dephasing timescales of the observed molecular modes and the duration and timing of the narrowband final pulse. Additionally, we discuss the virtues of a frequency-domain Lorentzian probe-pulse lineshape and its potential for improving the hybrid fs∕ps CARS technique as a diagnostic in high-pressure gas-phase thermometry applications.

Development of a simultaneously frequency- and time-resolved Raman-induced Kerr effect probe.

We detail the development of an optical probe technique based on time-resolved Raman-induced Kerr effect polarization spectroscopy (tr-RIKES). This technique, termed fs/ps RIKES, combines an ultrafast pump pulse with a narrowband probe that directly allows spectral resolution of low-frequency (0-600 cm(-1)) modes typically observable via RIKES. The narrowband probe pulse alleviates the need to scan the time delay between pump and probe pulses to observe molecular coherences, thus making this multiplexed technique a convenient probe for studying low-frequency molecular dynamics. An important distinguishing characteristic of this polarization-sensitive technique arises from the fact that the delay between the impulsive pump pulse and the picosecond-duration probe pulse is optimized to maximize suppression of nonresonant background signal. Model systems, including the rotational spectrum of gas-phase hydrogen and the low-frequency vibrational spectrum of neat bromoform, are used to compare fs/ps RIKES with the conventional time-resolved RIKES technique.

Capillary Extraction of the Ionic Liquid (Bmim)(DCA) for Variable Flow Rate Operations

The ionic liquid [Bmim][DCA] is a propellant candidate in a standalone electrospray thruster or in a dual-mode propulsion system consisting of a chemical system and an electrospray system. Since limited published data exists for [Bmim][DCA], the electrospray characteristics are relatively unknown. Emission testing of the ionic liquid has been conducted to characterize the [Bmim][DCA] electrospray plume for both an external flow titanium needle and internal flow capillary. Mass spectrometric, retarding potential, and angle distribution measurements were collected for the positive polarity ions emitted from [Bmim][DCA] wetted emitters with nominal extraction voltages between ~1 kV to ~2.5 kV. The titanium needle operated at a sizably reduced liquid flow rate in comparison to the capillary. As such, only the major species of Bmim + ([Bmim][DCA])n with n=0,1 were identifiable in the quadrupole measurement range of 0-1000 amu and were formed at or near the needle potential. A typical needle angle distribution was found in these measurements. For the capillary emitter, flow rates from 0.27 nL/s to 2.18 nL/s were used to investigate corresponding alterations in the electrospray beam. The aim of the investigation was to ascertain the ability to “tune” or “dial-in” an electrospray thruster to specific ion or droplet sizes and thus specific performance levels. Unlike the limited species observed from the needle emission, the capillary measurements indicated the presence of n=0,1,2,3,4 cation species with large mass droplet contributions. The lowest flow rates indicated the highest levels of ions in the measurement range of 0-1000 amu with a mix of large mass droplets. For increasing flow rate, species < 500 amu ceased to exist leaving only the n=2,3,4 species mixed with large mass droplets in the electrospray beam. All ion species exceeded the quadrupole mass range at the upper flow rates. Ions emitted from the capillary were formed at levels below the emitter potential. Ohmic losses in the ionic liquid are likely the cause for the less energetic ions. Angular distribution measurements indicated broadening of the beam current and mass distribution for increasing flow rates.

Molecular dynamics simulations of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide clusters and nanodrops.

Atomistic molecular dynamics simulations of small clusters and nanodroplets of the ionic liquid 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [EMIM-Tf2N] subject to an external electric field were performed. A 125-ion-pair droplet was found to be nearly spherical with an isotropic distribution of cations and anions under vacuum conditions. The droplet was subjected to external electric fields of varying strength, and ion emission events were observed. The initially spherical droplet is elongated along the electric field axis, resulting in nonspherical behavior and increased net dipole values after the application of strong electric fields. The critical electric field required for ion field emission was determined to be 0.985 V/nm, in agreement with the experimental value of 1.0 V/nm. Excellent agreement is found in the prediction of ionic emission products for a neutral 125-ion-pair droplet of the ionic liquid at an electric field strength of 1.2 V/nm when compared to the results of two independent experiments. Small ionic liquid clusters were investigated with respect to their thermal stabilities and were found to be thermally stable well above room temperature. The role of electric fields in the dissociation of small charged ion clusters was also investigated.

Krypton charge exchange cross sections for Hall effect thruster models

Following discharge from a Hall effect thruster, charge exchange occurs between ions and un-ionized propellant atoms. The low-energy cations produced can disturb operation of onboard instrumentation or the thruster itself. Charge-exchange cross sections for both singly and doubly charged propellant atoms are required to model these interactions. While xenon is the most common propellant currently used in Hall effect thrusters, other propellants are being considered, in particular, krypton. We present here guided-ion beam measurements and comparisons to semiclassical calculations for Kr+ + Kr and Kr2+ + Kr cross sections. The measurements of symmetric Kr+ + Kr charge exchange are in good agreement with both the calculations including spin-orbit effects and previous measurements. For the symmetric Kr2+ + Kr reaction, we present cross section measurements for center-of-mass energies between 1 eV and 300 eV, which spans energies not previously examined experimentally. These cross section measurements compare ...

Electrospray of an Energetic Ionic Liquid Monopropellant for Multi-Mode Micropropulsion Applications

The multi-mode chemical-electric propulsion capable energetic ionic liquid propellant [Emim][EtSO4]-HAN is electrosprayed in a 100 μm capillary emitter to test the electricmode performance of the propellant. The ionic liquid exhibits stable electrospray emission in both cation and anion extraction modes at a nominal extraction voltage of 3400 V. Near field measurements of current and mass flow rate distribution are taken at flow rates from 0.19 nL/s to 3.06 nL/s. Total emission current, as measured by Faraday cup and integrated, increases from 754 nA to 3195 nA for cation emission and from 552 nA to 2012 nA for anion emission. The thrust and specific impulse at 0.19 nL/s flow rate is 1.08 μN and 412 seconds, respectively, with a beam power of 2.22 mW. At 3.06 nL/s, the thrust is 8.71 μN and the specific impulse is 204 seconds with a beam power of 8.85 mW. Extrapolation of the current data shows that specific impulse in excess of 1000 seconds is achievable through optimized feed system and emitter design.

Species measurements in the beam of an ionic liquid ferrofluid capillary electrospray source under magnetic stress

Three solutions of an ionic liquid ferrofluid (ILFF) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-NTf2) as the carrier liquid were emitted from a capillary electrospray source and the resulting beam was measured using a time-of-flight mass spectrometer (TOF-MS). The solutions had 3.04, 5.98, and 8.80 wt% iron oxide nanoparticles making them susceptible to magnetic fields. A Helmholtz coil was used to impose a gradientfree magnetic stress onto the electrospray source. Mass spectra were collected in the spray from each of the solutions, with and without the imposed magnetic field. The magnetic stress caused an increase in the peak intensity of distinct ion species (n = 0 or n = 1) at lower energy defects suggesting that the stress causes ions to be born at higher energy. The ratio of the ion peak intensity with magnetic field to ion peak intensity at zero magnetic field was proportional to the concentration of nanoparticles. The magnetic stress did not significantly affect the large mass distributions until the nanoparticle concentration reached 8.80 wt% in the fluid.

Species measurements in the beam of an ionic liquid ferrofluid electrospray source

An ionic liquid ferrofluid (ILFF) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-NTf2) as the carrier liquid was emitted from the peak formed via the Rosensweig instability and the electrospray beam was measured using quadrupole and TOF mass spectrometry. The Rosensweig instability peak source (RIPS) was found to operate in three stable emission current modes: transient-emission, lowcurrent, and high-current. Both quadrupole and linear TOF mass spectra for the final two modes were collected, and revealed that the charged-particle species within the electrospray beam varied between the two emission current modes. No correlation between the magnetic field strength and the collected mass spectra was measured. Mass flow measurements using a quartz crystal microbalance revealed that the RIPS only operated at a high mass flow rate of 3-5 ng/s during the startup transient-emission mode and otherwise ran at mass flow rates of 0.03-0.3 ng/s indicating the absence of droplet species during the majority of emission. The RIPS quadrupole mass spectra were compared to a pure ionic liquid needle source quadrupole spectra and it was discovered that multiple species exist in the ILFF electrospray that are attributed to fragments respective ion species, with some species partially comprised of fragments from the polymer used for steric stabilization of the nanoparticles; specifically the block that comprises the stabilization group, poly(N,N-dimethylacrylamide), and the end functionalizing group, CH3CHCOOH.

Luminescence measurements of Xe+ + N2 and Xe2+ + N2 hyperthermal charge transfer collisions

Luminescence spectra are recorded for collisions between Xe(+)/Xe(2+) and molecular nitrogen at energies ranging from 4.5 to 316 eV in the center-of-mass frame. In the Xe(+) + N(2) collision system, evidence for luminescent charge-transfer products is only found through Xe I emission lines. The most intense features of the luminescence spectra are attributed to atomic N emissions observed above ∼20 eV. Intense N(2)(+) A (2)Π(u) - X(2)Σ(g)(+) and B(2)Σ(u)(+) - X(2)Σ(g)(+) radiance is observed from Xe(2+) + N(2) collisions. The B state formation cross section decreases with collision energy until 20 eV, after which it becomes independent of impact energy with an approximate value of 3 Å(2). The cross section for N(2) (+) A (ν > 0) formation increases with energy until 20 eV, after which it remains nearly constant at ∼1 Å(2). The N(2)(+) product vibrational distributions extracted from the spectra are non-Franck-Condon for both electronic product states at low collision energies. The distributions resemble a Franck-Condon distribution at the highest energies investigated in this work.

Mass spectrometry of selected ionic liquids in capillary electrospray at nanoliter volumetric flow rates

Capillary-based electrospray thrusters allow user-enabled control of the volumetric flow rate of the propellant. This control, coupled with mass spectrometry techniques spanning a large mass-to-charge range, enables elucidation of the composition of the electrospray beam and through further analyses, a better understanding of the physics occurring at the liquid/vacuum interface. In this work, mass spectra of selected ionic liquids electrosprayed from a capillary emitter are measured, using time-of-flight mass spectrometry, over a wide range of volumetric flow rates. The time-of-flight mass spectrometer enables simultaneous acquisition over a mass-to-charge range of 20 amu/q to ~500,000 amu/q in a single pulse cycle. Additionally, the use of orthogonal extraction enables direct determination of the kinetic energies of ions present in the electrosprayed beam. The presented data reveal a complex emission process occurring for ionic liquid capillary-based electrospray at nanoliter volumetric flow rates. The electrospray beam mass-to-charge composition includes a sequence of singly-charged ions and doubly-charged ions at nearly all sampled flow rates for all electrosprayed ionic liquids. In addition to the small ion-clusters, two Maxwell-Boltzmann distributions from approximately 10,000 amu/q to 500,000 amu/q exist in the spectra each with mass-to-charge distribution sensitive to the volumetric flow rate. The volumetric flow rate dependence of the largest charge-to-mass ratio droplets appears to be consistent with previously measured scaling laws, although exhibiting a wide mass-to-charge distribution. The flow rate dependence of the lower mass-to-charge distribution is more complex and is discussed in some detail. Measurement of the time-of-flight mass spectrum for a number of different ion kinetic energy defects over a number of different volumetric flow rates results in a more thorough understanding of kinetic energy losses experienced in the jet structure of the Taylor cone, with direct impacts to the thrust performance of these types of devices. The appearance of these ions and distributions alter the thought on the emission mechanism at work in the electrospray system. No longer is electrospray defined and easily explained by the Iribarne emission mechanism and droplet generation by the Rayleigh instability.